1. Field of the Invention
The present invention relates to a light source device in which clouding of optical parts is prevented, and to a method of producing the light source device.
2. Description of the Related Art
There has been a conventional light source device in which laser light emitted from a semiconductor laser element is transmitted through an optical fiber. In order to guide to the optical fiber the laser light emitted from the semiconductor laser element, the laser light emitted from the semiconductor laser element is first converted into parallel light using an optical lens, and then the parallel light is guided into the optical fiber by focusing using a condenser lens having a suitable focal length fitting with the optical-fiber characteristics (fiber diameter, NA (numerical aperture)). In this system, the energy level per photon under an ultra-violet-wavelength region (shorter than 400 nm) is an order (higher than 3 eV) equivalent to that of general chemical bonding. Because such short-wavelength light easily excites and dissociates impurities in air, the impurities excited and dissociated by the light adhere to neighboring optical parts, and thereby their transmittance is decreased; as a result, a harmful influence such as disorder of the optical wavefront of the laser light incident on an optical part is exerted. Such a problem has been first extensively highlighted in an industrial application, specifically in a light exposure device, for printing a fine circuit pattern on a silicon substrate, introduced in a production process of an ultra-large-scale integrated circuit (ULSI). In this device, in response to miniaturization of circuit patterns, light sources, outputting short-wavelength light, such as an i-line of a mercury lamp (wavelength of 365 nm), a KrF laser (that of 248 nm), and an ArF laser (that of 193 nm) have been applied one after another. In this system, ammonium sulfate ((NH4)2SO4) as a typical product is known to be produced by the following reaction equation using material such as sulfur dioxide (SO2) and ammonia (NH3) present in trace amounts in air being reacted with oxygen (O2) and water vapor (H2O) in air.2SO2+2H2O+O2→2H2SO4  (1)2NH3+H2SO4→(NH4)2SO4  (2)Ammonium sulfate produced causes transmittance decrease of the optical parts due to adhering to the optical parts.
Ultra-violet light is considered to activate SO2 in the reaction system indicated by the above equation (1), to accelerate the reaction, and the shorter the wavelength of the source light, the more remarkably the influence exerts on the speed of the product adhering to the optical parts.
Methods of improving the problems have been disclosed. A method is disclosed, for example, in Japanese Patent Publication No. 3266156 (page 4, FIG. 1), which is also published as U.S. Pat. No. 5,207,505, in which, by setting the temperature of an optical lens at higher than 120 degrees C. that is the sublimation temperature (decomposition temperature) of ammonium sulfate, the adhesion of ammonium sulfate is prevented. A method is disclosed, for example, in Japanese Patent Publication No. 3309867 (page 8, FIG. 1) in which, by using a reflection unit whose reflectance is set smaller with respect to light activating sulfur dioxide, the activation of sulfur dioxide is prevented. A method is disclosed, for example, in Japanese Patent Publication No. 3448670 (page 11, FIG. 1) in which, by forming an air curtain by flowing, for example, dry air or inert gas (N2, He, Ne or Ar, etc.) at the front of an optical part, sulfur dioxide, ammonia, water vapor being gaseous components constituting a reaction system are removed. A method is disclosed, for example, in Japanese Patent Application Publication Laid-Open No. 2004-259786 (page 9, FIG. 1), which is also published as United States Patent Application Publication No. 2004/0233401, in which, by devising a method of flowing the gas in the improvement method disclosed in Japanese Patent Publication No. 3448670, an impurity gas around the optical part is effectively removed. A method is disclosed, for example, in Japanese Patent Application Publication Laid-Open No. 2004-253783 (page 13, FIG. 1) in which, in an optical system for guiding laser light into an optical fiber, by hermetic sealing a semiconductor laser element and an optical lens for collimating the light, the semiconductor element and an optical part are isolated from the impurity gas. Moreover, a method is disclosed, for example, in Japanese Patent Application Publication Laid-Open No. 2003-306798 (page 9, FIG. 1) in which, regarding black plating treated on a lens tube for reducing scattered light, in order to reduce the amount of sulfate ions included in plating solution, by controlling a hot-water-washing time while monitoring the sulfate ions, the sulfate ions are controlled within a specified amount.
Various methods are disclosed in the above Patent Documents for preventing the clouding of the optical parts; however, measures have not been taken against the sulfur component included as an ingredient in the parts constituting the light source. However, the sulfur component included as an ingredient in the parts flows out of the parts, as described in later test results. In the devices disclosed in the above Patent Documents, it has not been considered that, due to the sulfur component flowing out of the parts, the amount of sulfate ions increases in the devices, and resultantly ammonium sulfate becomes easy to adhere to the optical parts.